A computerized tomography (CT) imaging apparatus operates by acquiring multiple 2D images with a rotating imaging ensemble or gantry that has an x-ray source and, opposite the x-ray source, an imaging sensor rotating about a fixed axis relative to the patient. CT imaging allows the reconstruction of 3D or volume images of anatomical structures of the patient and is acknowledged to be of particular value for obtaining useful information for assisting diagnosis and treatment.
There is considerable interest in the use of CT imaging in dental and ear-nose-throat (ENT) applications, as well as for other imaging of the patient's head. A number of volume imaging system designs have been proposed for this purpose. Among proposed solutions are hybrid systems that combine panoramic imaging and CT imaging. For example, U.S. Pat. No. 6,118,842 entitled “X-RAY IMAGING APPARATUS” to Arai et al. discloses an X-ray imaging apparatus that supports both imaging modes. The apparatus includes an X-ray source, an X-ray detector for detecting X-rays having passed through the subject, and supporting means for supporting the X-ray source and the X-ray detector so that they are spatially opposed to each other across the subject; and mode switching means for switching between a CT mode and a panorama mode. To detect X-rays, only one large area X-ray detector is used. The X-ray imaging apparatus can obtain both types of images by switching modes during the imaging session. However, the proposed imaging apparatus requires an expensive detector capable of carrying out both imaging functions in a satisfactory manner. Additionally, systems of this type typically compromise image quality by using a uniform distance between the X-ray source and detector, even though different distances would be more advantageous.
By way of example, FIG. 1 shows an embodiment of a conventional CT imaging apparatus 40. A column 18 is adjustable for height of the subject. The patient 12 or other subject, shown in dotted outline, is positioned between an x-ray source 10 and an x-ray imaging sensor panel 20, also termed an imaging detector. X-ray imaging sensor panel 20 rotates on a rotatable mount 30 in order to position a CT sensor 21 for obtaining the exposure. CT sensor 21 is positioned behind the subject, relative to x-ray source 10. The operator rotates CT sensor 21 into this position as part of imaging setup. With rotation of mount 30, sensor 21 and source 10 revolve about the head of the patient, typically for some portion of a full revolution. Still other dental imaging system solutions combine CT, panoramic, and cephalometric imaging from a single apparatus. With such combined systems, the required amounts of radiation exposure can be a concern, particularly for CT imaging, which can require numerous images, each from a separate exposure.
Conventional digital radiography detectors have some limitations related to how attenuation of radiation energy at a single exposure is interpreted. For example, it can be very difficult, from a single exposure, to distinguish whether an imaged object has a given thickness or a given attenuation coefficient. To resolve this ambiguity, some systems provide separate, sequential low-energy and higher energy exposures and use the resulting difference in image information to distinguish between types of materials. However, in order to provide this information, this type of imaging requires that the patient be subjected to additional radiation for the second exposure. This problem can be compounded for CT imaging, in which multiple images are obtained, one from each of a number of angles of revolution about the patient.
Conventional CT imaging provides useful information that aids in diagnosis and treatment, but is constrained by limitations of the imaging sensor apparatus itself and by concerns over exposure levels needed for obtaining the desired image quality. Thus, it can be seen that there is a long-felt need for improved methods of imaging that can achieve high levels of image quality with reduced exposure and at more favorable cost for dental, ENT, and other imaging of the head.